A bypass turbine engine generally comprises, from upstream to downstream in the gas flow direction, a shrouded fan, an annular primary flow space and an annular secondary flow space. The air mass aspired by the fan is therefore divided into a primary flow F1, which circulates in the primary flow space, and a secondary flow F2, which is concentric with the primary flow F1 and circulates in the secondary flow space.
The primary flow space passes through a primary body comprising one or more compressor stages, for example a lowpressure compressor and a highpressure compressor, a combustion chamber, one or more turbine stages, for example a highpressure turbine and a lowpressure turbine, and a gas exhaust nozzle.
Moreover, in a manner known per se, the turbine engine comprises an intermediate casing the hub whereof is arranged between the low pressure compressor casing and the highpressure compressor casing. The intermediate casing comprises discharge valves or VBV, the role of which is to regulate the flow rate at the inlet of the highpressure compressor so as, in particular, to limit the risks of surge in the low pressure compressor by discharging a portion of the air outside the primary flow space.
As illustrated in FIG. 1, which is a partial axial section view of a dual-spool bypass airplane turbojet of a known type, the hubs 2 of the intermediate casings usually comprise two coaxial annular shrouds, respectively inner 3 and outer 5, mutually connected by an upstream transverse flange 7 and a downstream transverse flange 9.
The upstream flange 7 is arranged downstream of the lowpressure compressor while the downstream flange 9 is arranged upstream of the highpressure compressor.
The inner shroud 3 delimits the annular primary flow space 10 of the primary flow F1 of the turbine engine and comprises air inlet openings 4 distributed circumferentially around an axis X of the inner shroud 3 (which is coaxial with the hub 2), which are blocked by a corresponding discharge valve 12 designed to regulate the flow rate of the highpressure compressor.
Such a discharge valve 12 can take the form of a door which is mounted pivotally on the inner shroud 3 between a closed position, in which the door 12 closes the corresponding inlet opening 4 and is flush with the inner shroud 3 of the intermediate casing 1 while forming a substantially continuous surface to best reduce the risks of aerodynamic perturbations of the primary flow F1, and an open position (see FIG. 1), in which the door 12 protrudes radially toward the inside with respect to the inner shroud 3 and thus allows the collection of a portion of the primary flow F1 in the primary flow space 10.
For its part, the outer shroud 5 delimits the secondary flow space 14 of the secondary flow F2 of the turbine engine, and comprises air outlet openings 6 arranged downstream of the downstream transverse flange 9 and distributed circumferentially around the axis X.
When the airflow rate which can enter the highpressure compressor is reduced, a surplus of air in the secondary flow space 14 can then be bled through these outlet openings 6, thus avoiding surge phenomena which can lead to deterioration or complete destruction of the lowpressure compressor.
The turbine engine further comprises discharge streams, formed between the inlet openings 14 and the outlet openings 6. Each discharge stream is delimited, from upstream to downstream, between an inlet opening 4 and an associated outlet opening 6, by an intermediate annular space 16, delimited by the shrouds 3, 5 and the transverse flanges 7, 9, then by a discharge stream duct 18 (also known by the acronym kit engine), configured to guide the air flow to the secondary flow space 14. The discharge stream duct 18 further comprises an intermediate opening 19, which leads into the intermediate space 16 at the upstream surface of the downstream transverse flange 9.
The doors 12, the intermediate spaces 16 and the associated discharge stream ducts 18 thus form together a system for discharging air to the secondary flow space 14 of the turbine engine.
The hub 2 of the intermediate casing 1 therefore includes a plurality of such systems distributed around the axis X.
Moreover, when a door 12 of a discharge valve is in the open position, an air flow scooped by it passes through the intermediate space 16, the corresponding discharge stream duct 18, then reaches the secondary flow space 14 through a discharge grating 20 comprising fins, or the VBV grating. The discharge streams and the fins of the VBV gratings 20 are inclined with respect to the flow direction of the secondary flow F2, so as to redirect the air flow from the primary flow space and align it as much as possible with that of the secondary flow F2.
Modern turbine engines operate at ever greater dilution ratios (better known as bypass ratios). In order to limit shock losses in supersonic flows at the tip of the fan, the angular rotation speed of the fan is reduced. This has the effect of reducing the compression ratio of the fan. At lower compression ratios, the head and secondary flow F2 separation losses therefore have a greater impact and must be limited as much as possible. These head losses are present in the zone having surface irregularities in particular.
The Applicant, however, has noted the fact that the presence of the VBV grating 20 created a stream irregularity capable of create head losses when the discharge stream is not discharging (i.e. when the door 12 of the discharge valve is in the closed position), typically during cruise. In fact, the VBV grating 20 forms a porous surface into which air can enter and capable of create head losses and/or separation layer in the secondary flow F2.
Proposed therefore, in document FR 15 52811, filed 1 Apr. 2015 in the Applicant's name, is an intermediate casing hub for a bypass turbine engine comprising:                a set of discharge fins, attached in the discharge stream duct, at the outlet opening of the outer shroud, and        blocking means, configured to adjust a passage cross-section of the outlet opening depending on the position of the movable door.        
The blocking means are movable between an open configuration, in which an air flow from the inlet opening is able to pass through the discharge fins, and a closed configuration, in which the blocking means block a passage cross-section of the outlet opening. These blocking means can in particular be formed by discharge fins which are then mounted pivotally in the discharge stream duct between the open configuration and the closed configuration.
However, these blocking means require the implementation of servo control means and therefore the addition of components into the engine and therefore the increase of its mass. Typically, in patent application FR 15 52811, the coupling is accomplished by means of a digital control system or a servo-control system mechanically or hydraulically connecting the door to the blocking means and thus ensuring their simultaneous opening and closing.
Document US 2013/269366 describes an intermediate casing hub comprising a discharge stream duct which leads into a secondary flow space through an outlet opening and discharge fins, the opening and the closing of the discharge fins being synchronized by means of a pivoting rod.